This invention relates to non-rising stem type gate valves and, more specifically, to such a gate valve having a low stress structure connecting the stem to the valve gate to facilitate opening and closing of the gate valve when it is subjected to high fluid pressures of approximately 5000 psi and above.
Gate valves, both slab type and expanding type, are used in the pipeline industry to control fluid flow and as such are sometimes subjected to high operating pressures (e.g. 5000 psi and above) and fluid flow velocities. One problem occurring when gate valves are used in high fluid pressure installations is the increased thrust and torque requirements encountered when the gate member of the valve is moved between its open and closed positions. It has been found that gate members are deflected as much as 1/4.degree. (1/32" to 1/16") from the longitudinal axis of the stem/gate member structure when gate valves are used in these environments. If gate valves are used in sour gas services such as hydrogen sulfide, the stress effects produced can result in hydrogen embrittlement and hydrogen corrosion cracking of the gate member. Thus, there is a need to provide a low stress stem connection between the valve stem and gate member in a non-rising stem type gate valve that accommodates high fluid pressures.
U.S. Pat. No. 3,463,446 issued to Natho describes a low stress stem connection for a rising stem type gate valve in which the problems described above are eliminated or at least their effects reduced. However, this specific structure is not applicable to a non-rising type gate valve due to the operational differences therebetween. In a rising stem gate valve the stem is fixedly connected to the gate and moves in a direction along its longitudinal axis upon the rotation thereof. Essentially, the gate and valve stem do not move relative to one another in a direction along the longitudinal axis of the stem. In a non-rising stem type gate valve the stem is fixed within the valve body so that upon rotation the stem does not move in a direction along its longitudinal axis. The stem is threadedly connected to the gate member in a fashion so that the rotation of the stem will cause the gate to move relative to the stem in a direction along the longitudinal axis of the stem. U.S. Pat. No. 243,846 issued to Buick, U.S. Pat. No. 659,234 issued to Hughes and U.S. Pat. No. 2,726,842 issued to Seamark are exemplary of the non-rising stem type valve.
The U.S. Pat. No. 3,223,380 issued to Hochmuth et al. shows a stem-gate connection in a non-rising stem type gate valve. However, excessive material must be removed from the extension in the gate member of the structure shown in U.S. Pat. No. 3,223,380 so that the extension, and consequently, the width of the gate, must be made thicker so as to possess the requisite strength to accommodate higher fluid pressures. If it is not thickened, it cannot accommodate higher fluid pressures. Although the requisite strength can be achieved by increasing the thickness of the gate member, a thick gate member is not desirable from a manufacturing standpoint. This is especially true in expanding gate valves where the total thickness of the gate and segment is an important manufacturing consideration. An expanding gate valve is illustrated in U.S. Pat. No. 4,116,419 to Diehl et al. It would therefore be highly desirable to provide a low stress stem connection structure for connecting a valve stem and a gate member for a non-rising stem type gate valve that has a gate of an acceptable thickness and that can accommodate high fluid pressures.